1. Field of Invention
The field of the present invention relates in general to communication systems and more particularly to digital to analog conversion systems for the transmit path of an XDSL modem.
2. Description of the Related Art
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation. Currently there are over ten discrete X-DSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc.
One of the factors limiting the bandwidth or channel capacity of any of the above discussed X-DSL protocols is distortion. The components on the transmit path of a modem inherently distort signals as they transmit them. Amplitude modulation causes distortion to become dependent on the input signal with a result of the amplified output signal is no longer simply an amplified replica of the input signal. Unfortunately if linear modulation with a fluctuating envelope is used in conjunction with nonlinear amplification, spectral spreading may occur thereby interfering with communications.
What is needed are approaches to reducing in band distortion for X-DSL modems.
An apparatus and method is disclosed for minimizing in band distortion in the transmit path of an XDSL modem. The transmit path of an XDSL modem introduces various non-linearities into transmissions in both the frequency and time domains. The current invention provides a means for both determining and correcting for distortion in the time domain. Distortion in the time domain may result from digital to analog conversion or analog filtering for example. The apparatus may be incorporated in an existing X-DSL architecture without additional circuitry. The apparatus may be applied with equal advantage in wired and wireless media.
In an embodiment of the invention the apparatus may include a calibrator which may be implemented using the existing analog-to-digital (ADC) conversion and demodulation capabilities on the receive path of the modem or alternately, a dedicated module. During a calibration phase a training signal is injected digitally into the digital-to-analog converter of the transmit path. The analog portion of the transmit path is coupled to a calibrator which measures the corresponding analog response. The correlation between input and response is stored in an analog model.
During subsequent training and run-time phases of modem operation an oversampled digital input stream is subject to digital Delta Sigma Modulation (xcex94xcexa3M) with the analog model injected into the feedback path from the quantizer. The injection of the analog model into the feedback improves the noise transfer function (NTF) associated with the xcex94xcexa3M.